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US researchers identified 305 women who had been diagnosed with MS during a three-year period.

Their use of the Pill – mainly a combination of two hormones – was compared with 3,050 women who did not have MS.

In total, 29 per cent of the women
with MS and 24 per cent of those without MS had used hormonal
contraceptives for at least three months in the three years before
symptoms began.

Women who had used the Pill were 35 per cent more likely to develop MS than those who did not use them. Those who had used the contraceptives but had stopped at least one month before symptoms started were 50 per cent more likely to develop MS.

Lead researcher Dr Kerstin Hellwig said: ‘These findings suggest that using hormonal contraceptives may be contributing at least in part to the rise in the rate of MS among women.’

Previously animal research led experts to believe female hormones might delay the onset of MS, and a British study suggested Pill users had a 40 per cent lower risk.

The new research was presented at the American Academy of Neurology’s annual meeting in Philadelphia.

In a separate study at the same meeting, researchers looked at a possible link between obesity and MS, by checking the Body Mass Index (BMI) of study volunteers.

BMI was calculated for 210 people with MS and 210 people of the same age and sex who did not have MS at ages 15 and 20 and at the time of the study.

The study found that people who are obese at age 20 are twice as likely to later develop MS as people who are not obese.

The study found that people with higher BMI levels also had higher levels of leptin, a hormone made by fat tissue that regulates weight, appetite and immune response.

Study author Dr Jorge Correale, of the Raúl Carrera Institute for Neurological Research in Buenos Aires, Argentina, said: ‘Leptin promotes inflammatory responses in the body, which could potentially explain the link between obesity and MS.’

Dr Hellwig said MS was a relatively rare disease, but most likely to strike women between the ages of 20 and 40 – when they would be taking the pill.

She said: ‘We don’t say it causes MS, but adjusting for other factors more women use the pill who develop MS.'

‘There may be some environmental factor that we have not been able to allow for, we are not telling women to stop using the pill,’ she added.